Various passenger protection devices such as an airbag and a seatbelt pretensioner are equipped in vehicles recently. The passenger protection system including such a passenger protection device includes, as shown in FIG. 1, front sensors 11a, 11b mounted at both front left and right sides of a vehicle 10, safety sensors 13a, 13b mounted at a front or rear passenger seats in a passenger compartment, and a plurality of sensors (first to third sensors) 15a to 15c, 16a to 16c mounted at both left and right sides of the vehicle 10. These sensors are connected to an electronic control unit (ECU) 18, thus forming a communications network. Each of the sensors 11a, 11b, 13a to 13c, 15a to 15c, 16a to 16c detects travel speed or collision of the vehicle, and the ECU 18 activates airbags (not shown) based on the detected travel speed or collision.
In this communications network, the sensors 15a to 15c, 16a to 16c have respective switches in the inside parts and connected to the ECU 18 through buses. These switches are closed sequentially by initialization of setting addresses from the sensor closest to the ECU 18, when electric power is supplied in the vehicle 10. Specifically, the switch of the first sensor 15a, which is closest to the ECU 18, is set with its address and closed to connect the second sensor 15b to the ECU 18. After setting an address to the sensor 15b by the ECU 18, the switch of the sensor 15b is closed to connect the sensor 15c as the third sensor to the ECU 18. The initialization is performed in this order.
In the communications between the ECU 18 and each sensor 15a to 15c, 16a to 16c, voltage communications is performed from the ECU 18 to each sensor 15a to 15c, 16a to 16c and current communications is performed from each sensor 15a to 15c, 16a to 16c to the ECU 18.
In the voltage communications, for example, “0” and “1” are used. “0” is an amplitude signal, if one-third (⅓) is 0 volt (V) and two-thirds (⅔) is 5 V with respect to the duty. “1” is an amplitude signal, if one-third (⅓) is 5 V and two-thirds (⅔) is 0 V with respect to the duty. Here, the duty is the ratio of time of 5V relative to one cycle time of the signal.
In the current communications, for example, “0” and “1” are used as well. However, “0” is a current signal of 0 milliamperes (mA) and “1” is a current signal of 10 mA.
It is assumed that the sensors 15a to 15c and the ECU 18 are bus-connected to each other through a power-side line 21 and a ground-side line 22 in series or in sequence as shown in FIG. 2. The sensors 15a to 15c have respective capacitors 24a to 24c at sides of the ECU 18 (input sides), and switches 26a to 26c to connect to the sensor of the following stage.
The voltage communications from the ECU 18 to each sensor 15a to 15c is indicated by (a) and the current communications from each sensor 15a to 15c to the ECU 18 is indicated by (b) in FIG. 3.
It is first assumed that the switches 26a to 26c of the sensors 15a to 15c are all in the off-state. It is also assumed that the capacitor 24a of the first sensor 15a is charged in response to a first charge command of the ECU 18 in the idle phase before time t1. In the following signal phase from time t1 to time t2, the ECU 18 transmits to the first sensor 15a a first command to set a first address by the voltage communications. The first sensor 15a closes its switch 26a after receiving the first command and setting the first address, so that the second sensor 15b is connected to the ECU 18 therethrough.
The ECU 18 transmits a second charge command in the following idle phase from time t2 to time t3. The capacitor 24b of the second sensor 15b is charged in response to the second charge command. In the signal phase from time t3 to time t4, the first sensor 15a transmits to the ECU 18 a first response, which indicates completion of setting of the first address. The ECU 18 transmits a second command of second address setting to the second sensor 15b by the voltage communications. The second sensor 15b closes its switch 26b after receiving the second command and setting the second address, so that the third sensor 15c is further connected to the ECU 18 therethrough. After receiving the first response from the first sensor 15a, the ECU 18 performs communications with the first sensor 15a by using the first address, which is included in the first response from the first sensor 15a. 
When the ECU 18 transmits a third charge command in the following idle phase from time t4 to t5, the capacitor 24c of the third sensor 15c is charged. In the signal phase from time t5 to time t6, the second sensor 15b transmits to the ECU 18 a second response, which indicates completion of setting the second address. The ECU 18 transmits a command of third address setting to the third sensor 15c by the voltage communications. The third sensor 15b sets its address after receiving the third command. After receiving the second response from the second sensor 15b, the ECU 18 performs communications with the second sensor 15b by using the second address, which is included in the second response from the second sensor 15b. 
In the similar manner, the capacitor 24c of the third sensor 15c is charged in the idle phase from time t6 to time t7. Then the third sensor 15c transmits to the ECU 18 a third response, which indicates completion of setting the third address, in the next signal phase from time t7 to time t8.
JP 2007-215102A (U.S. Pat. No. 7,539,804) also discloses a conventional communications network, in which an ECU communicates with sensors by setting respective addresses in the similar manner as described above. According to the conventional communications networks, the ECU communicates with the sensors at communications speed of 150 to 200 kbps in the signal phase. In this instance, higher harmonics (noises) of frequencies corresponding to several times of the communications speed are generated, thus adversely affecting AM (amplitude modulation) radio frequency band of 510 kHz to 1710 kHz. Increased cost is necessitated to reduce such noises.